Similar zone-center gaps in the low-energy spin-wave spectra of NaFeAs and BaFe2As2

TL;DR

This study compares low-energy spin-wave spectra in NaFeAs and BaFe2As2, revealing similar zone-center gaps despite differences in magnetic moments and transition temperatures, and providing insights into their magnetic interactions.

Contribution

It demonstrates that two structurally distinct iron-pnictide compounds have similar spin gaps, challenging previous assumptions about their magnetic interactions.

Findings

Both compounds exhibit ~10 meV spin gaps at the magnetic zone center.

The spin gap opens sharply below the Née temperature with no precursor gap.

Weak out-of-plane dispersion suggests a smaller effective interlayer exchange interaction.

Abstract

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na(1-{\delta})FeAs and BaFe2As2. Despite their very different values of the ordered magnetic moment and N\'eel temperatures, T_N, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T_N, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na(1-{\delta})FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe2As2 along the out-of-plane momentum component, q_z. At the magnetic zone boundary (q_z = 0), spin excitations in the ordered state persist down to 20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J_c, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.